In recent years, automatic tack hammers which receive tacks from a hopper and drive them at high speeds have become known. Areas in which such devices have not developed wide acceptance, however, is in the field of tacking decorative nails into upholstered furniture and screw driving devices. Such decorative nails are generally round-headed, which create problems both in the feeding control, and also in the alignment of the nail once it is in the chamber of the automatic hammer. For examples of automatic tack hammers and feeders therefor, attention is directed to U.S. Pat. Nos. 2,994,880, 3,283,985, and 3,305,155.
One of the problem areas resides in the escape mechanism which removes fasteners one at a time from a delivery slide and drops them into a pneumatic conduit for delivery individually to the hammer. At the outlet of the escape mechanism, it is known that the fasteners may be dropped by means of gravity into the conduit. Since the initial movement of the fastener responsive to the gravity is relatively slow, this considerably slows down the operation of the mechanism. If the mechanism is attempted to be operated too fast, the escape mechanism will return to a closed position and again engage the fastener head before it has had a chance to drop through the outlet opening In order to combat this problem, previously known devices have introduced a jet of air upstream from the discharge outlet of the escape mechanism to force the fastener out of the escape mechanism fastener, however this also presents a problem, previously unsolved, in that when a jet of air is introduced upstream from the escape mechanism outlet, some of the air blows back through the escape mechanism into the delivery slide which may cause misfeeding of the fasteners, and again slows down the feeding procedure.
A further problem that must be considered is that the tacks must be fed head first, because otherwise the point of the tack may snag in the wall of the flexible conduit and jam the apparatus. Since the fastener is delivered headfirst, it must be returned to the point first position in the drive mechanism.
It is apparent that the shank of the fastener must be coaxially aligned with the longitudinal axis of the receiving chamber. Otherwise, the drive piston will urge the fastener into the workpiece at an angle, causing bending of the nails or screws or improper fastening. A previously known solution for this has been the use of resilient "dogs" on the side of a barrel which engage and grip the head of the fastener and maintain it in proper alignment, however such dogs protrude outwardly from the barrel and prevent access of the gun or hammer to places where access is necessary.
Since air is being used to feed fasteners into the hammer, the fasteners may be inadvertently discharged through the front end of the barrel during forward movement of the drive piston unless the air pressure can be selectively deactivated during the step. Also, the air that delivers the fastener into the receiving chamber must be released, and if vented properly can be used to advantage.